聚噻吩太陽能電池總體異質接面層之形態控制

Morphology Control of Bulk Heterojunction Layer in Polythiophene-based Solar Cells

張怡鳴(Yi-Ming Chang)

指導教授 ： 王立義

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.02451

摘要

高分子太陽能電池中異質接面層之形態為決定元件性能之重要關鍵，為求得一奈米尺度之電子施體/受體雙連續相交錯互穿網路式結構，提升異質材料間之相容性或使用適當之退火處理乃為一必要之異質接面形態最佳化程序，而本研究之主要貢獻在於成功開發出數種可針對高分子太陽能電池異質接面層形態進行調控之技術，並加以探討之。論文第一部份主要驗證共軛高分子，聚己烷噻吩[poly(3-hexylthiophene), P3HT]於大氣環境遭受光致氧化/劣化效應作用後，材料之有效共軛長度、光吸收/放射能力、結晶性與載子位移率等光電特性均會產生改變，進而造成異質接面形態與元件性能不盡理想，強調在有機光電元件發展上，若欲最佳化元件之品質與性能，除製程技術與製備環境之提升外，共軛高分子合成、純化程序與儲存環境亦須顧及，以確保材料品質，避免影響元件性能；論文第二部份則首先合成出一經由官能基改質而成之poly[3-hexylthiophene-co-3-(6-hydroxyhexyl) thiophene] (P3HT-OH)共軛高分子，再搭配原位溶凝膠反應與titanium (IV) n-butoxide進行一系列共軛高分子:二氧化鈦混成材料之製備。由於共軛高分子中hydroxyl基團的導入可明顯增加高分子與二氧化鈦間之交互作用，進而使二氧化鈦可均勻分散於高分子基質中，形成均勻之混成溶液與薄膜。而此混成材料可進一步作為一界面相容物質添加於P3HT與二氧化鈦中，形成具有優異掺混性之共軛高分子:二氧化鈦異質接面薄膜。相對於未添加該混成材料之異直接面薄膜而言，由該分散均勻之薄膜所製備之太陽能電池在光電流與能量轉換效率的表現上分別具有3.4倍與2.4倍之顯著增長，成功驗證了此一材料製備概念之可行性；而第三部份我們成功發展出一套利用外加直流電，於20秒之短暫時間對P3HT與 [6,6]-phenyl C61-butyric acid methyl ester (PCBM)所製備之太陽能電池進行異質接面形態修飾之技術。經由此直流電處理後之太陽能電池，其元件性能可有明顯的提升。而利用空間電荷限制電流與電化學阻抗分析對元件進行量測後可知，此方式可大幅改善P3HT:PCBM異質接面層之載子位移率以及總體電阻，且元件之能量轉換效率與穩定性完全不亞於傳統熱退火處理之太陽能電池，證明此一簡便、快速之製程可有效應用於提升高分子太陽能電池性能。最後，我們製備出一高效率不需退火處理之高分子異質接面太陽能電池，其方式係利用微量之3-hexylthiophene (3HT)作為成膜輔助溶劑，即添加少量P3HT之單體於P3HT與PCBM混合溶液中，並以簡易之旋轉塗佈法製備異質接面薄膜。利用如此薄膜所製作之太陽能電池可明顯地提升其光電流與有效輸出功率，而不再需要任何熱退火或是溶劑慢速揮發等處理程序。在利用紫外光-可見光吸收光譜、X-ray繞射光譜與穿透式電子顯微鏡對該免退火元件之異直接面薄膜進行細部探究後可知，3HT之輔助成膜有助於促進異質接面層中P3HT之光吸收度與結晶性增加，並且達到一理想之奈米尺度相分離形態。最重要的是此一免退火P3HT:PCBM太陽能電池在能量轉換效率與光致電子轉換效率表現上與一般熱退火處理元件相當，因此相信此一技術將來極具潛力可有效且廣泛地用於高分子太陽能電池發展。

關鍵字

退火 ； 異質接面 ； 共軛高分子 ； 形態 ； 光伏元件 ； 高分子太陽能電池

並列摘要

The morphology of bulk-heterojunction layer is an important key in determining the performance of polymer-based solar cells. In order to obtain a nano-scaled bi-continuous interpenetrating network of electron donor/acceptor blend, desirable phase compatibility or annealing treatment is essential procedures for morphology optimization. Therefore, considerable efforts have been devoted to this study on controlling the morphology of bulk-heterojunction layer in the polymer-based solar cells. In the first part, we demonstrated that the light illumination of regioregular poly(3-hexylthiophene) (P3HT) in air resulted in the degradation of polymer, thus potentially reducing the conjugation length, the absorption/emission intensities, the size of the crystal domain and the charge-carrier mobility. These effects lead to undesirable morphology and poor performance of solar cells. Therefore, the development of efficient polymer solar cells depends not only on fabrication in an inert atmosphere, but also on the synthesis and storage of material in a suitable environment to ensure polymer quality. In the second part, a series of poly[3-hexylthiophene-co-3-(6-hydroxyhexyl)thiophene] (P3HT-OH):titania (TiO2) hybrids were synthesized via the in-situ sol-gel reaction of titanium (IV) n-butoxide in the presence of P3HT-OH. Introducing hydroxyl moiety onto the side chain of polymer significantly promotes the polymer-titania interaction, producing a uniform distribution of TiO2 throughout the polymer matrix in the hybrid solutions and hybrid films. Furthermore, this hybrid can be further utilized as an efficient compatibilizer in preparing photoactive P3HT:TiO2:hybrid films with excellent miscibility. The solar cell based on such a hybrid exhibited the 3.4 and 2.4-fold higher value of photocurrent and power conversion efficiency compared to the device based on P3HT:TiO2 blend without added hybrid. In the third part, we successfully demonstrated an novel electric annealing technique for fabricating efficient P3HT:[6,6]-phenyl C61-butyric acid methyl ester (PCBM) solar cells by applying a direct current in a short annealing time for only 20 sec. The solar cells based on this approach exhibited distinctly enhanced device performance. The results from space-charge limited current measurement and electrochemical impedance analysis indicated a significant improvement in the charge-carrier mobility and bulk resistance for thus-prepared P3HT:PCBM bulk-heterojunction films. The power conversion efficiency and stability of this solar cell was comparable to that of conventional thermally annealed devices. This approach provides an operationally simple and rapid process for improving the solar cell efficiency. Finally, an annealing-free solar cell fabrication technique based on the addition of small concentration of 3-hexylthiophene, the monomer of P3HT into the P3HT and PCBM blend solution from which the bulk-heterojunction layer was prepared by spin-cast was demonstrated. The polymer solar cells based on such films exhibited distinctly enhanced photocurrent and fill factor without any thermal annealing or solvent annealing process. The results from UV-vis absorption spectra, X-ray diffraction spectra, and TEM images indicated a significant improvement in the absorbance, crystallization, and nano-scaled morphological optimization for thus-prepared P3HT:PCBM bulk-heterojunction films. Very importantly, the solar cells fabricated from this annealing-free process performed similarly in power conversion efficiency and incident photo-to-electron conversion efficiency as conventionally annealed solar cells, indicating the feasibility of this simple and rapid annealing-free fabrication process.

並列關鍵字

Annealing ； Bulk-heterojunction ； Conjugated Polymer ； Morphology ； Photovoltaic Devices ； Polymer Solar Cells

參考文獻

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